Porsche has finally revealed the full specs of the 918 Spyder at the Volkswagen Group night ahead of the Frankfurt Motor Show.

The 918 Spyder sports a 4.6-liter V8 engine for 608 horsepower. It also has two electric motors (a 154 HP electric motor turns the rear axle and a 127 HP electric motor spins the front wheels) added into the mix as well, for a total system of 887 HP and 590 lb-ft of torque.

The 918 Spyder is a two-seater constructed of carbon fiber, and it weighs only 3,692 pounds.

As for its hybrid abilities, the 918 Spyder can travel on electric power at speeds up to 93 MPH and get about 10 to 20 miles per charge. Charging options include a German 230-volt outlet, which can do the job in about four hours. A DC fast charger will be optional and can recharge a battery in just 25 minutes.

The hybrid features a liquid-cooled lithium-ion battery consisting of 312 individual cells with an energy content of approximately 7 kilowatt hours.

The 918 Spyder has four driving modes, including the standard E-Power for an electric-only range of up to 18 miles at speeds up to 93 MPH; Hybrid mode, which gives the most efficient power delivery of up to 85 MPG; Race Hybrid, which increases gear ratios spinning the electric motors while throttling up the V8, and Hot Lap, which pushes the traction battery to its maximum power output limits for a few fast laps.

Carrying extra fuel instead of the battery's weight would be a rule favoring non-hybrids.

OTOH if you'd just set up rules with minimum weight and maximum fuel, a hybrid can have an advantage. Sure it is heavier, but some 200+ extra hp for a few seconds and/or reduced fuel consumption can give it the edge.

tl;dr: "Sure it is heavier" is non-trivial and shouldn't just be hand-waved away.

We're veering into 'Internet Argument' land and should maybe just agree to disagree, but I'll try to articulate my point a bit better.

In a road car, using a hybrid powertrain has a few engineering (as opposed to marketing) tradeoffs: you gain better fuel economy (manifested in lower non-acquisition costs), primarily due to regenerative breaking, and an arguably preferable power band, at the cost of money and weight.

In a high-end racecar the monetary penalty of hybrids isn't a big deal. The weight penalty however is critical. It's why racecars are much lighter than road cars, and why any time there isn't a specified car/component weight max racecar teams move towards more and more advanced materials. Fuel adds weight of course, but it's pretty light. A few pounds will get you around the track at race speed once the engine is already in place. A hybrid system won't do that - you need a few HUNDRED pounds. You could remove the gas engine entirely, but then your races would be about 15 minutes long.

In your scenario you mention a spec of minimum weight and max fuel. IF you set that weight high enough (quite a bit higher than needed to make a safe and reliable racecar) then your hybrid system makes sense, because the race team is going to need to run ballast anyways, so heavy dense batteries might as well replace heavy dense blocks of lead (you end up with a bit worse center of gravity, but that's a minor point). But that's a pretty arbitrary set of rules - if you let the race teams run as much fuel as they want they're going to carry more fuel instead of batteries; on the other hand if you let them make the cars as light as they want they're going to run lighter cars and either refuel more often or opt to produce less power. Letting the teams opt to carry X weight in fuel or battery isn't a non-hybrid favoring rule, it's a performance-favoring rule, no different than letting teams chose what tire compound or aerodynamic package to use.

I also know some racing hybrids use their batteries for more power rather than more range, but that's even worse. Now you're carrying a few hundred pounds extra for roughly 3hp per second of each lap. That's simply a godawful return on your weight budget compared to running a larger engine and just burning more fuel.